A typical elevator system includes an elevator car which is disposed to be moved up and down in an elevator shaftway adjacent to a plurality of floor landings. The motion of the car is controlled by a sheave motor and a brake, in response to commands thereto provided by a car controller which is usually mounted at the head of the shaftway, adjacent to the sheave, motor and brake. The car controller causes the motor to raise and lower the elevator, to decelerate and accelerate it, and to stop it at floor landings. The car controller generally keeps track of the position of the car in the shaftway, and the hall calls and car calls which are to be responded to. The car controller also notifies the cab controller when the car is stopping to allow passenger transfer, so that the cab controller can open the doors, and generally tells the cab controller when the doors have been open long enough and commands that the doors again be closed. The cab controller, in some cases, may assist in the leveling of the car by providing detailed information with respect to the position of the car with respect to an adjacent landing.
Communications between the cab controller (mounted on the car) and the car controller (mounted in a machine room) has typically either been by direct wire connections or employing time division multiplex over a few wires, the wires in either case being in what is typically called the traveling cable between the elevator car and the machine room. Where direct wiring is used for every signal, the car controller and cab controller typically employ mechanical switches and magnetic relays, which are inherently difficult to maintain. On the other hand, where multiplexing is employed, the failure of one or two devices in the electronic circuitry which handles the multiplexing may result in a failure of communication between the car and the cab. Whenever there is a communication failure between the cab controller and the car controller, the car controller is unable to command the doors to be opened and/or closed in a proper fashion. In prior systems, this has typically resulted in the passengers being trapped in the car, and being released only after the intervention of maintenance personnel. This is typically true even though the car may be standing at a landing so that the passengers could escape safely if the commands could be provided to open the door.